Abstract. We investigated the impact of mineral dust particles on clouds, radiation and atmospheric state
during a strong Saharan dust event over Europe in May 2008, applying a comprehensive
online-coupled regional model framework that explicitly treats particle microphysics and chemical
composition. Sophisticated parameterizations for aerosol activation and ice nucleation, together
with two-moment cloud microphysics are used to calculate the interaction of the different
particles with clouds depending on their physical and chemical properties.

The impact of dust on cloud droplet number concentration was found to be low, with just a slight
increase in cloud droplet number concentration for both uncoated and coated dust. For
temperatures lower than the level of homogeneous freezing, no significant impact of dust on the
number and mass concentration of ice crystals was found, though the concentration of frozen dust
particles reached up to 100 l−1 during the ice nucleation events. Mineral dust particles
were found to have the largest impact on clouds in a temperature range between freezing level and
the level of homogeneous freezing, where they determined the number concentration of ice crystals
due to efficient heterogeneous freezing of the dust particles and modified the glaciation of mixed
phase clouds.

Our simulations show that during the dust events, ice crystals concentrations were increased
twofold in this temperature range (compared to if dust interactions are neglected). This had
a significant impact on the cloud optical properties, causing a reduction in the incoming
short-wave radiation at the surface up to −75 W m−2. Including the direct
interaction of dust with radiation caused an additional reduction in the incoming short-wave radiation by 40 to
80 W m−2, and the incoming long-wave radiation at the surface was increased significantly in the order of
+10 W m−2.

The strong radiative forcings associated with dust caused a reduction in surface temperature in
the order of −0.2 to −0.5 K for most parts of France, Germany, and Italy during the dust
event. The maximum difference in surface temperature was found in the East of France, the Benelux,
and Western Germany with up to −1 K. This magnitude of temperature change was sufficient to explain
a systematic bias in numerical weather forecasts during the period of the dust event.